1. Field of the Invention
The present invention relates to a technique to drive a liquid jet unit for jetting droplets from a liquid surface, and more particularly to a technique to jet droplets by providing vibration to a liquid having a surface limitedly exposed by an opening.
2. Description of the Background Art
There has been a technique of depositing ink onto printing paper in a form of droplet to draw images and characters, i.e., ink jet printing. In this description, for distinction, a term "jet" is used when a plurality of ink droplets are simultaneously generated and a term "discharge" is used when droplets are sequentially generated one by one.
To faithfully reproduce tone of an image proposed is a technique of ultrasonically providing ink with vibration to obtain droplets from an ink surface. For example, Japanese Patent Application Laid Open Gazette No. 2-303849 discloses a technique of controlling the length of period for continuously providing ink with ultrasound to control the amount of ink to be discharged. Japanese Patent Application Laid Open Gazette No. 10-128968 discloses a technique of providing ink with ultrasonic burst including a certain number of pulses repeatedly a plurality of times to control the amount of ink to be jetted with the repeat number.
In the technique of controlling the length of period for continuously providing ink with ultrasound to control the amount of ink to be discharged, however, in order to discharge a large amount of ink, it is necessary to provide ink with continuous ultrasonic supply for a long time. That causes a marked rise of the level of ink surface from the opening with a radiation pressure as discussed later, resulting in an unstable discharge of droplets.
The technique of providing ink with ultrasonic burst repeatedly a plurality of times is superior in not providing continuous radiation pressure. Since the ink jet printing, however, requires depositing of ink on printing paper with different tones for dots, it is necessary to adjust the timing of sending the printing paper and that of disposing ink for every dot. The length of period while no continuous ultrasonic is supplied varies from dot to dot in the technique of providing ink with ultrasonic burst repeatedly a plurality of times as well as in the technique of controlling the length of period for continuously providing ink with ultrasound. Since the length of period while a radiation pressure is provided therefore varies from dot to dot, the rise of ink level from the opening varies from dot to dot. This variation causes an unstable jetting of droplets, and further causes deterioration in graininess in terms of printing quality and difficulty in controlling gradation.